This invention relates generally to roller cutters of the type mounted on cutter heads of tunnelling or earth-boring machines and more particularly to the mounting of such roller cutters so that they may be removed and replaced from the rear of the cutter head.
Widely used forms of tunnelling machines utilize a radially extending circular cutter head having a diameter equal to that of the tunnel being bored, which is mounted on a supporting frame for rotation about an axis coincident with or parallel to the axis of the tunnel itself. The driving mechanism for rotating the cutter head generally includes one portion which moves axially along with the cutter head and a second portion which is clamped to the tunnel walls so that thrust cylinders between the two portions of the frame work force the rotating cutter head against the tunnel face.
In the case of hard rock and many forms of softer rock, it has been found that the most effective cutter is a roller having a sharp edge or a plurality of hard buttons which compressively engage the rock to cause it to chip and break away in relatively small pieces. Roller cutters, whether of the disk or button type, are generally arranged to roll about an axis which extends parallel to the face of the cutter head and intersects the axis of rotation of the cutter head. While the diameter of cutters may vary somewhat, depending upon the diameter of the cutter head, it is more conventional to increase the number of cutters on larger heads while distributing them over the face of the cutter head in such a way as to balance the thrust forces acting between the cutter head and the adjacent tunnel face while locating them in a pattern that insures that every point on the entire tunnel face will be engaged by at least one cutter during one single rotation of the cutter head.
While such arrangements have been used for many years, they face a problem whenever a cutter becomes damaged or inoperative and must be replaced. Obviously, the larger the cutter head and the more cutters, the greater the likelihood of cutters needing replacement during any given period of operation. The replacement of cutters has been a problem in the past where earlier designs had roller cutters which were mounted on the front face of the cutter head in such a manner that they could be reached for repair or replacement only from the front side of the cutter head. Thus, if service is required on any of the individual cutters, it is necessary to reverse the machine to move the cutter head away from the tunnel face a sufficient distance to provide working room and it is often necessary to have a portion of the cutter head that is removable to allow people and equipment to have access to the front face of the cutter head. As a result, this not only increases the cost of design and manufacture of the cutter head, but also may result in increased cost because of the long down time required for service of this type.
When cutters were mounted on the front face of the cutter head, the mounting was usually accomplished by welding a massive U-shaped saddle member directly to the cutter head with a saddle having a pair of upstanding portions that would mount the ends of a fixed axle or journal assembly on which the cutter wheel is mounted. At one time, tunneling machines of this type were arranged to rotate in only a single cutting direction and this allowed constructions such as those shown in U.S. Pat. Nos. 3,749,188 and 3,851,718, which allowed simplified removal and replacement by a mounting structure that was arranged to take force in one direction only. However, it has been found that because of the tendency of the fixed frame to move and rotate in a direction opposite the direction of the cutter rotation because of the reaction forces from driving the cutter head, newer machines are designed to rotate in either direction so that by periodically changing the direction of rotation of the cutter head, the tendency of the fixed frame to walk around the tunnel is substantially eliminated. However, this required redesign of the cutters to take thrust forces in both directions and typical arrangements are shown in U.S. Pat. Nos. 3,791,705 and 3,863,994.
More recently, it has been proposed that roller cutters can be mounted on a cutter head in such a manner that they can be removed and replaced from the rear face of the cutter head so that the cutter head need not be moved backward from the tunnel face, thereby greatly decreasing the time the tunnelling machine is out of service for cutter repair or replacement. The problem has been, however, to find a suitable means of supporting the roller cutter in place in view of the very high forces involved during cutting. When the cutter is mounted on the front, this is no problem since the end housings on the saddle can be made quite large in size to support in compression the housing ends of the roller assembly frame on which the roller is journalled. If the assembly is merely reversed in direction, then fasteners such as bolts and the like would then be placed in tension which is not satisfactory.
One solution to this problem has been shown in U.S. Pat. No. 4,202,418, granted May 13, 1980, which provides a stationary frame having openings on both the front and rear sides of the cutter head. The cutter assembly includes a stator to support the actual rolling cutter, and the stator is arranged to be inserted into the rear opening and supported by stops near the front opening through which a portion of the cutting edge can project. To hold the stator assembly in position, a plurality of wedge blocks are inserted against each end of the stator and held in place by individual bolts. Because the wedge blocks are inserted perpendicular to the axis of the roller cutter and bear against abutment surfaces on both the stator ends and the frame, the thrust forces on the roller cutter are transmitted through the wedge blocks.